Genetic differentiation of bicolor damselfish (Eupomacentrus partitus) populations in the Florida Keys

Electrophoretic variation in proteins encoded by 23 loci revealed substantial genetic differentiation among populations of bicolor damselfish (Eupomacentrus partitus) collected from four coral reefs in the Florida Keys, USA, during 1986–1988. Genetic differentiation was concentrated between a sample collected from Little Grecian Rocks Reef (LGR) and the remaining samples, including fish from a reef only 600 m distant (Grecian Rocks Reef). Genetic distinction of the LGR sample derived from significantly heterogenous allelic frequencies at six of eight polymorphic loci. Aco-1 (aconitase); Ada (adenosine deaminase); Gpi-2 (glucosephosphate isomerase); Ldh-2 and Ldh-3 (lactate dehydrogenase); and Me-1 (malic enzyme); nevertheless, differentiation at cytosolic aconitase (Aco-1) far exceeded that observed for other loci (fixation index, F _ST=0.482), and differences in Aco-1 allele frequencies were largely responsible for large genetic distances (0.20) between LGR and the other reefs. Paradoxically, estimates of numbers of migrants exchanged between reefs per generation (mN _e=17.47) indicated the potential for extensive gene flow. The extent of genetic differentiation among these populations is evaluated relative to models of population genetic structure based on equilibrium between gene flow and natural selection or genetic drift.     

Genetic structure of dictyoceratid sponge populations on the western Coral Sea reefs

Allozyme variation at six polymorphic loci was examined in foliose dictyoceratid sponges from isolated reefs in the western Coral Sea. Four major genetic groups corresponding to the species Phyllospongia lamellosa, P. alcicornis, Carterospongia flabellifera and Collospongia auris were examined. A further two rare morphotypes from individual reefs formed genetic outliers to the P. lamellosa group, and may represent further taxa related to P. lamellosa. Gene frequencies in individual reef populations were largely in Hardy-Weinberg equilibrium, suggesting that random mating occurred in local populations of all four common species. Genetic variability was high and observed heterozygosities within populations ranged from 0.13 to 0.40. All four taxa showed significant genetic differentiation among populations (F _ST=0.05 to 0.36). Genetic distances (Nei's D) among populations within species ranged from 0 to 0.723 and increased with increasing geographical separation. There was evidence that genetic differentiation between populations to the north and to the south of the southern limit of the South Equatorial Current (SEC) divergence was greater than expected on the basis of their geographical separation. The SEC divergence may form a partial barrier to gene flow among populations of these ecologically important sponges on the submerged Queensland Plateau. Levels of migration among populations of three of the species was less than those required to prevent divergence of the populations through genetic drift (Nm<1). Restricted migration among populations may provide a mechanism to explain the occurrence of highly divergent populations of dictyoceratid sponges whose specific identity is not clear, and may allow them additionally to develop partial reproduction isolation from other populations.     

Variation in the genetic composition of coral (Pocillopora damicornis and Acropora palifera) populations from different reef habitats

The genetic structure of populations of the corals Pocillopora damicornis and Acropora palifera was examined in three habitats at One Tree Island during March and April 1993, using electrophoretically detectable variation at six allozyme loci. There were significant genetic differences among populations of P. damicornis within each of the reef crest, lagoon and microatoll habitats. The level of differentiation among populations was similar in each of the habitats. Differences between populations of P. damicornis from lagoon and microatolls were no greater than that within habitats, but genetic differentiation of these from crest populations was much higher. There was no difference in the genetic composition of A. palifera populations within or between the lagoon and microatolls, the only habitats where this species was found. Both coral species had observed:expected (G _O:G_E) genotypic diversity rations >0.80, indicating predominantly sexual reproduction. These data, the high genotype diversity and general conformance of genotype frequencies to those expected under conditions of Hardy-Weinberg, suggested panmixis at each site. The high degree of sexual reproduction in the P. damicornis populations is unusual for a species where asexual reproduction has been the dominant mode of reproduction reported to date. Gene flow in both species was considerable between the lagoon and the closed microatolls. The genetic differences between populations of P. damicornis in these habitats and the reef crest may reflect the relative isolation of all populations within the closed One Tree Lagoon from those outside. However, local currents appear to offer effective means of dispersal between the habitats, suggesting that the genetic differences result from natural selection in the different environments within One Tree Lagoon and the reef crest.     

Genetic structure of the scleractinian coral<Emphasis Type="Italic"> Platygyra sinensis</Emphasis> in Hong Kong

Genetic structure was studied in Platygyra sinensis from six sites in Hong Kong by allozyme electrophoresis using an isoelectric focusing gel (IEF). Seven variable loci were detected using six enzyme systems. The number of alleles per locus ranged from 3.3 to 3.6 in the six samples, and heterozygosity from 0.429 to 0.540. Genotype frequencies were generally close to Hardy–Weinberg expectations. The values of unique multilocus genotype to the number of individuals for each sample (N_G/N) and observed genotypic diversity to expected genotypic diversity (G_O/G_E) were high, indicating a high level of sexual reproduction. Variation in allele frequencies among sites was low (F_ST=0.017), as was Nei's unbiased genetic distance (D), suggesting genetic similarity among sites.Communicated by T. Ikeda, Hakodate     

Daily feeding rates in herbivorous labroid fishes

Estimates of daily feeding rates were obtained for two groups of herbivorous labroid fishes, one confined to cold water and the other to tropical reef environments. These were the family Odacidae, represented by Odax pullus from New Zealand waters, (Goat Island Bay: Latitude 36° South; on the northeastern coast of New Zealand) and the family Scaridae, represented by Scarus rivulatus, S. schlegeli and S. sordidus from the northern Great Barrier Reef (Lizard Island; a mid-shelf reef at 14° South latitude). Observations on the odacid were made in 1984 and in 1992, and on the scarids in 1984 and 1988. O. pullus displayed a diurnal feeding pattern in which the rates (expressed as bites min^-1) are greatest early in the day. The mean combined feeding rate for three size groups (juveniles, subadults and adults) peaked (average of 2.9 bites) from 06.00 to 08.00 hrs and declined fourfold to a combined average of 0.7 bites min^-1 by midday. The greatest mean feeding rate recorded was 3.7 bites min^-1, with an overall mean of 1.8 bites min^-1. For subadults and adults there were consistent trends in feeding, with subadults feeding at a greater rate than adults and both groups displaying a decline in feeding rate during the day. The change in feeding rate with time of day was statistically significant in both groups. The pattern for juvenile O. pullus was different from that in the two larger size groups in that juveniles did not show a uniform decline in feeding with time of day. For scarids, the daily feeding rate varied by site, but the pattern was similar for all species, characterised by initial low rates increasing to higher but variable levels by midday. The influence of both site of feeding and time of day on feeding rate was confirmed by analysis. The overall mean values for each species were 20.1 bites min^-1 for S. rivulatus, 19.7 bites min^-1 for S. schlegeli and 14.9 bites min^-1 for S. sordidus. For scarids, the peak feeding rates varied from 19.3 to 32.8 bites min^-1, with overall rates from 14.9 to 21.1 bites min^-1. Estimates of activity and movement patterns during feeding were obtained for O. pullus. Distance moved per unit time was highly variable, 0.1 to 47.5 m min^-1, with a mean of 8.5 m min^-1 (SD=9.9). Trends in movement among sexes and size classes were obscured by the variable movement patterns of individual fishes.     

Experimental effect of cold,La Niña temperatures on the survival of reef fishes from Gorgona Island (eastern Pacific Ocean)

The eastern tropical Pacific (ETP) reefs are affected at irregular times by extremely cold temperatures that occur principally during La Niña events. The effects of these low temperatures on the survival of reef fishes were experimentally assessed by determining the critical thermal minimum (CTM) of 15 reef fish species from Gorgona Island (ETP), and comparing these CTMs with the records of temperature during past La Niña events. Among species, mean CTMs ranged from 10.8°C to 16.3°C, which were lower than the coldest temperature recorded during the last La Niña event (18°C during La Niña 1998-1999). However, the observed ranges of CTM for two species (Thalassoma lucassanum and Eucinostomus gracilis) extended above 18°C. These results suggest that most of the reef fishes we studied are physiologically tolerant to the cold temperatures encountered during La Niña, though decreases in at least two populations may be expected as a result of the mortality of less tolerant individuals. Although tolerant to cold temperatures, reef fish populations may still experience negative changes during La Niña, because other determinants in population maintenance (e.g. reproduction and recruitment) are more temperature sensitive. The effects of other cold phenomena on reef fish survival are also discussed herein.     

Short-term growth measurements of corals using an accurate buoyant weighing technique

An accurate method for determining the growth rates of the skeleton of isolated branch tips (nubbins) of corals over intervals of less than 24 h is described. The skeletal weight of the coral was estimated from its buoyant weight in seawater whose density had been accurately determined. The coral tissues accounted for between 1 and 5% of the total buoyant weight in Pocillopora verrucosa and Acropora humilis with differing relative tissue biomass. After correcting for tissue buoyant weight, predictions of skeletal weight were accurate to within 1%. The method was used to estimate the growth of sample nubbins of Porites porites of similar diameter, in 2 m of water at Discovery Bay, Jamaica. Since growth of these branch tips is apical, growth rate could be expressed without correction for the size. The mean 24 h skeletal growth rate ranged between 40 and 47 mg. Differences could be measured between day-time and night-time growth, the day: night ratio being 3.7. The method also showed that P. porites virtually ceases calcification during the 4 to 5 d periods that it becomes enclosed in a mucus tunic. Nubbins of P. porites attached to the reef at different locations showed clear differences in growth rate with depth, and between clear and turbid water sites. The growth rate of nubbins was compared with that of branch tips of whole corals by measuring the linear extension after staining with Alizarin Red S. After 3 1/2 mo, the mean linear extension was 4.1 mm in each case, indicating that the growth rate of nubbins is the same as that of branch tips of the whole colony. It is suggested that this buoyant weighing technique will find applications in laboratory experiments with calcification mechanisms and as a bioassay on reefs exposed to environmental stress.Contribution No. 464 of the Discovery Bay Marine Laboratory     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Genetic evidence for two sibling species within Polydora cf. ciliata (Polychaeta: Spionidae) from the Sea of Japan

Two morphologically indistinguishable but genetically very distinct polychaete species of the family Spionidae are found in the Peter the Great Bay of the Sea of Japan. Both species bore into molluscan shells and occur sympatrically. They are close in morphology to Polydora ciliata (Johnston, 1838) and some other members of the redefined P. ciliata species complex. The systematics of the two species is still under consideration by one of us (V.I.R.). A total of nine enzyme systems comprising 12 isozyme loci were examined in both the species by means of starch gel electrophoresis. Nei's genetic distance between them proved surprisingly high (D= 1.942) being the highest among estimates so far obtained for pairs of polydorid species. The levels of intraspecific genetic variation in both species (Polydora sp. 1, H _e= 0.287 ± 0.064; Polydora sp. 2, H _e= 0.111 ± 0.045) are comparable to those of other electrophoretically studied polydorids. The problem of taxonomic integrity of cosmopolitan marine invertebrate species is briefly discussed as well as the usefulness of isozymes as species-specific characters suitable for unequivocal discrimination between sibling polydorid species. A list of enzymes characterized by interspecifically variable numbers of expressed isozymes within polydorids is given and recommended for such a discrimination. Received: 22 July 1997 / Accepted: 19 February 1998     

Annual biomass and production of the oceanic copepod community off Discovery Bay,Jamaica

Monthly samples were collected in oceanic waters off Discovery Bay, Jamaica, in 60- and 200-m vertical hauls, using 200- and 64-m mesh plankton nets, from June 1989 to July 1991. Length-weight regressions were derived for twelve genera of copepods (R²=0.79 to 0.97). For eight occasions spanning the study period, biomass estimates generated from these length-weight regressions differed by only 3% from direct weight determinations. The mean ash content of copepods was 7.1%, and the energy density was 20.8 kJ g^-1 ash-free dry weight (AFDW). Mean annual biomass of the total copepod community in the upper 60 m was 1.83 mg AFDW m^-3 (range 1.14 to 2.89 mg AFDW m^-3), and for the 200-m water column was 0.96 mg AFDW m^-3 (range 0.12 to 1.99 mg AFDW m^-3). Estimates of generation times for five common taxa ranged from 16.1 to 33.4 d. None of the taxa investigated displayed isochronal development; in general, stage duration increased in later copepodite stages. Weight increments showed a significant decrease in later copepodite stages, but with strong reversal of the trend from stage 5 to adult female in most species. Daily specific growth rates also declined in later copepodite stages, and ranged from 1.49 d^-1 in stage 1–2 Paracalanus/Clausocalanus spp. to 0.04 in stage 5-female of Oithona plumifera. Progressive food limitation of somatic copepodite growth and egg production is postulated. Naupliar production was 50.4 to 59.5% of copepodite production, and egg production was 35.1 to 27.7% of copepodite production in the 60-and 200-m water columns, respectively. Total annual copepod production, including copepodites, nauplii, eggs and exuviae, was 160 kJ m^-2 yr^-1 for the upper 60 m and 304 kJ m^-2 yr^-1 for the upper 200 m. Secondary production of the copepod community in oceanic waters off Discovery Bay approaches 50% of the corresponding value in tropical neritic waters.     

Genetic structure of Atlantic and Gulf of Mexico populations of sea bass,menhaden, and sturgeon: Influence of zoogeographic factors and life-history patterns

To assess the influence of zoogeographic factors and life-history parameters (effective population size, generation length, and dispersal) on the evolutionary genetic structure of marine fishes in the southeastern USA, phylogeographic patterns of mitochondrial DNA (mtDNA) were compared between disjunct Atlantic and Gulf of Mexico populations in three coastal marine fishes whose juveniles require an estuarine or freshwater habitat for development. Black sea bass (Centropristis striata), menhaden (Brevoortia tyrannus andB. patronus) and sturgeon (Acipenser oxyrhynchus) samples were collected between 1986 and 1988. All species showed significant haplotype frequency differences between the Atlantic and Gulf, but the magnitude and distribution of mtDNA variation differed greatly among these taxa: sea bass showed little within-region mtDNA polymorphism and a clear phylogenetic distinction between the Atlantic and Gulf; menhaden showed extensive within-region polymorphism and a paraphyletic relationship between Atlantic and Gulf populations; and sturgeon exhibited very low mtDNA diversity both within regions and overall. Evolutionary effective sizes of the female populations (N _f (e)) estimated from the mtDNA data ranged fromN _f (e) = 50 (Gulf of Mexico sturgeon) toN _f (e) = 800 000 (Atlantic menhaden), and showed a strong rank-order agreement with the current-day census sizes of these species. The relationship betweenN _f (e) and the estimated times of divergence (t) among mtDNA lineages (from conventional clock calibrations) predicts the observed phylogenetic distinction between Atlantic and Gulf sea bass, as well as the paraphyletic pattern in menhaden, provided the populations have been separated by the same long-standing zoogeographic barriers thought to have influenced other coastal taxa in the southeastern USA. However, vicariant scenarios alone cannot explain other phylogenetic aspects of the menhaden (and sturgeon) mtDNA data and, for these species, recent gene flow between the Atlantic and Gulf coasts is strongly implicated. These data are relevant to management and conservation issues for these species.Please address all requests for reprints to Dr. J. C. Avise     

Substratum preferences in planula larvae of two species of scleractinian corals, Goniastrea retiformis and Stylaraea punctata

To test whether coral planulae recruit randomly to different coral reef habitats or have specific substratum preferences, the settling behavior of planulae from two shallow water coral species from Pago Bay, Guam (13°25.02N, 144°47.30E) were examined in the laboratory in June and July of 1995. Goniastrea retiformis is generally restricted to the shallow reef front (<10 m depth) in areas dominated by crustose coralline algae (CCA), while Stylaraea punctata is abundant on inner reef flats were CCA coverage is low and sand and carbonate rubble covered by biofilms is common. When presented with four substrata (1) carbonate rock scrubbed free of biofilm and dried as a control, (2) the CCA Hydrolithon reinboldii, (3) the CCA Peyssonelia sp., and (4) naturally conditioned carbonate rubble covered by a biofilm, G. retiformis larvae showed a significant preference for H. reinboldii, and S. punctata larvae for the carbonate biofilm treatment. The preference shown by S. punctata larvae for biofilmed surfaces did not diminish with increasing larval age up to 11 days. These results suggest that the larvae of both species are capable of habitat selection, and that the preferred substrata among those tested bears a relationship to the habitats in which adult colonies were found. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Resident mysids: community structure,abundance and small-scale distributions in a coral reef lagoon

Seasonal and diel variations in community structure and abundance of coral-reef lagoon mysids were examined at Davies Reef in the central region of the Great Barrier Reef (GBR) between June 1980 and May 1981. Twenty-five mysid species belonging to three subfamilies of the family Mysidae were captured during the study, with six new records for the GBR. The epibenthic mysid community differed from that in the overlying water, was faunistically uniform, but formed characteristic seasonal and diel groupings. The dominant epibenthic species were Erythrops sp., Anisomysis pelewensis, Doxomysis littoralis, A. laticauda, Prionomysis stenolepis, A. lamellicauda, and A. australis, five of which formed schools. Total mysid abundances ranged between 110 and 790 m^-3 with peak abundance in October. Schooling species occurred at local densities of up to 500 000 m^-3. Mysids were absent from shallow and midwater depths during the day, but were distributed throughout all depths at night with peak abundances in mid-water and deep layers. The dominant species in the water column at night were Pseudanchialina inermis, A. laticauda and Gastrosaccus indicus, in descending order of abundance. Lagoonal mysids contribute little to the food of sessile reef planktivores, as all but three species remain concentrated near or on the lagoon floor both day and night. The contribution of resident lagoon mysids to reef trophodynamics is probably through remineralization of lagoon detritus. Given the vast reef areas comprised of sandy lagoons, the large populations and relatively large size of lagoon mysids, this trophodynamic role may be of considerable importance.A.I.M.S. Contribution No. 477     

The relationship between population genetic structure and pelagic larval duration in coral reef fishes on the Great Barrier Reef

Pelagic larval duration (PLD) is a commonly used proxy for dispersal potential in coral reef fishes. Here we examine the relationship between PLD, genetic structure and genetic variability in geographically widespread and ecological generalist species from one coral reef fish family (Pomacentridae) that differs in mean larval duration by more than a month. The genetic structure was estimated in eight species using a mitochondrial molecular marker (D-loop) and in a sub-set of five species using nuclear molecular markers (ISSRs). Estimates of genetic differentiation were similar among species with pelagic larvae, but differed between molecular markers. The mtDNA indicated no structure in all species except one, while the ISSR indicated some structure between the sampling locations in all species. We detected a relationship between PLD and genetic structure using both markers. These relationships, however, were caused by a single species, Acanthochromis polyacanthus, which differs from all the other species examined here in lacking a larval phase. With this species excluded, there was no relationship between PLD and genetic structure using either marker despite a range of PLDs of more than 20 days. Genetic diversities were generally high in all species and did not differ significantly among species and locations. Nucleotide diversity and total heterozygosity were negatively related to maximum PLD but again these relationships were caused by A. polyacanthus and disappeared when this species was excluded. These genetic patterns are consistent with moderate gene flow among well-connected locations and indicate that at this phylogenetic level (i.e., within family) the duration of the pelagic larval phase is unrelated to the patterns of genetic differentiation.     

Extreme genetic diversity and temporal rather than spatial partitioning in a widely distributed coral reef fish

Mitochondrial control region (HVR-1) sequences were used to identify patterns of genetic structure and diversity in Naso vlamingii, a widespread coral reef fish with a long evolutionary history. We examined 113 individuals from eight locations across the Indo-Pacific Ocean. Our aims were to determine the spatial scale at which population partitioning occurred and then to evaluate the extent to which either vicariance and/or dispersal events have shaped the population structure of N. vlamingii. The analysis produced several unexpected findings. Firstly, the genetic structure of this species was temporal rather than spatial. Secondly, there was no evidence of a barrier to dispersal throughout the vast distribution range. Apparently larvae of this species traverse vicariance barriers that inhibit inter-oceanic migration of other widespread reef fish taxa. Thirdly, an unusual life history and long evolutionary history was associated with a population structure that was unique amongst coral reef fishes in terms of the magnitude and pattern of genetic diversity (haplotype diversity, h = 1.0 and nucleotide diversity π = 13.6%). In addition to these unique characteristics, there was no evidence of isolation by distance (r = 0.458, R ² = 0.210, P = 0.078) as has also been shown for some other widespread reef species. However, some reductions in gene flow were observed among and within Ocean basins [Indian–Pacific analysis of molecular variance (AMOVA), Φ _st = 0.0766, P < 0.05; West Indian–East Indian–Pacific AMOVA Φ _st = 0.079, P < 0.05]. These findings are contrasted with recent studies of coral reef fishes that imply a greater degree of spatial structuring in coral reef fish populations than would be expected from the dispersive nature of their life cycles. We conclude that increased taxon sampling of coral reef fishes for phylogeographic analysis will provide an extended view of the ecological and evolutionary processes shaping coral reef fish diversity at both ends of the life history spectrum.     

The influence of contrasting life history traits and oceanic processes on genetic structuring of rabbitfish populations Siganus argenteus and Siganus fuscescens along the eastern Philippine coasts

Genetic variability and structuring of rabbitfish populations with contrasting life histories, Siganus argenteus and Siganus fuscescens were determined using allozyme analysis. A total of 13–14 polymorphic loci were examined from samples collected in 2002 and 2003 from eight reefs representing 25 populations north (Kuroshio Current) and south (Mindanao Current) of the bifurcation of the North Equatorial Current along the eastern Philippine coast. S. fuscescens populations (H _OBS = 0.085) showed higher heterozygosity than S. argenteus (H _OBS = 0.053), consistent with predictions of the neutral theory for demersal egg spawners compared to pelagic egg spawners. The generally lower genetic variability of Kuroshio populations may be due to greater environmental disturbance affecting larval mortality and recruitment success. There was no significant overall population genetic structuring for S. argenteus (F _ST = 0.01485, P > 0.05) compared to S. fuscescens (F _ST = 0.03275, P < 0.05). The latter species showed highly significant genetic structuring among Kuroshio and Mindanao Current populations in both 2002 and 2003 (F _CT = 0.08120, P < 0.05; F _CT = 0.07500, P < 0.05, respectively), as well as among populations within regions. This conforms to expectations of correlations between observed population genetic structure and life history features related to dispersal potential and gene flow. However, there were significant temporal (i.e., 2002 vs. 2003 samples) genetic variations for both S. fuscescens (F _CT = 0.08542, P < 0.05) and S. argenteus (F _CT = 0.06330, P < 0.05), which may reflect interannual variability in recruitment success. Differences in population spatial genetic patterns between the two reef fish species suggest that broad scale physical factors (e.g. NEC bifurcation) and regional environmental perturbations (e.g. incidence of typhoons) affect population genetic structure of sympatric congeneric species with different life histories differently. Finer scale ecological processes, which affect larval dispersal and recruitment (e.g., local hydrographic features, distribution of habitats), particularly in the Mindanao Current region, exert more influence on structuring populations of S. fuscescens.     

Genetic population structure of the recently introduced Asian clam,Potamocorbula amurensis,in San Francisco Bay

The genetic population structure of the recently introduced Asian clam, Potamocorbula amurensis, in San Francisco Bay was described using starch gel electrophoresis at eight presumptive loci. Specimens were taken from five environmentally distinct sites located throughout the bay. The population maintains a high degree of genetic variation, with a mean heterozygosity of 0.295, a mean polymorphism of 0.75, and an average of 3.70 alleles per locus. The population is genetically homogeneous, as evidenced from genetic distance values and F-statistics. However, heterogeneity of populations was indicated from a contingency chi-square test. Significant deviations from Hardy-Weinberg equilibrium and heterozygote deficiencies were found at the Lap-1 locus for all populations and at the Lap-2 locus for a single population. High levels of variability could represent a universal characteristic of invading species, the levels of variability in the source population(s), and/or the dynamics of the introduction. Lack of differentiation between subpopulations may be due to the immaturity of the San Francisco Bay population, the general purpose phenotype genetic strategy of the species, high rates of gene flow in the population, and/or the selective neutrality of the loci investigated.     

Genetic structure of populations of two closely related brittle stars with contrasting sexual and asexual life histories,with observations on the genetic structure of a second asexual species

Ophiocoma pumila Lütken andOphiocomella ophiactoides (H. L. Clark) are morphologically similar brittle stars with contrasting life histories, the former obligately sexual, the latter fissiparous (capable of both sexual reproduction and asexual proliferation by binary fission). Electrophoretic analysis of five polymorphic enzymes was used to assess the genetic consequences of these differing life histories and provide a genetic perspective on the taxonomic relationship between the two species. Genotypic diversity ofOphiocoma pumila collected at Discovery Bay, Jamaica, in 1985 conformed to expectations for a sexually reproducing population. In contrast, genotypic diversity ofOphiocomella ophiactoides at this site was significantly lower than expected for a sexually reproducing population, due largely to the predominance of clonal proliferation over larval recruitment. Large variation in clonal composition over a short (50 m) distance emphasized the very localized scale of clonal mixing in this species. Allozymic data are indicative of a close sibling species relationship betweenOphiocoma pumila andOphiocomella ophiactoides which suggests that the present generic separation of the two species should be re-examined. Electrophoretic analysis was also used to examine the genetic structure of sponge- and alga-dwelling populations of a second fissiparous brittle star,Ophiactis savignyi (Müller & Troschel), which was also collected at Jamaica in 1985. Striking differences in the allelic composition of sponge- and alga-dwellingO. savignyi were observed. Genotypic diversity ofO. savignyi in sponges was very low, each sponge being dominated by a single genotype. Genotypic diversity ofO. savignyi in algae was higher, although still significantly lower than expectations for a sexually reproducing population. In the light of the highly clonal composition of fissiparous brittle-star populations, the adaptive significance of clonal growth may be related to an increase in the overall fitness of dispersed clones (genets), compared to individuals of strictly sexual counterparts, through greater genotypespecific biomass and, hence, fecundity.Contribution No. 476 of the Discovery Bay Marine Laboratory of the University of the West Indies     

Marine diatoms grown in chemostats under silicate or ammonium limitation. IV. Transient response of Chaetoceros debilis,Skeletonema costatum,and Thalassiosira gravida to a single addition of the limiting nutrient

The kinetic response of ammonium- or silicate-limited and ammonium- or silicatestarved populations of Chaetoceros debilis, Skeletonema costatum, and Thalassiosira gravida was determined by a single addition of the limiting nutrient to a steady-state culture and subsequent monitoring of the nutrient disappearance of the limiting and non-limiting nutrients at frequent time intervals. The kinetic response of nonlimited (nutrient) populations of these three species was also determined. Three distinct modes of the uptake of the limiting nutrient were observed for ammonium-or silicate-limited populations of these three species, surge uptake (V _s ), internally (cellular) controlled uptake (V _i ), and externally (ambient limiting nutrient concentration) controlled uptake (V _e ). Non-limited populations did not exhibit the three distinct segments of uptake, V _s , V _i and V _e . Estimates of the maximal uptake rate (V _max) and the Michaelis constant (K _s ) were obtained from nutrient-limited populations during the V _e segment of the uptake curve. Pooled values of V _e for the three ammonium-limited populations yielded V _max and K _s estimates of 0.16 h^-1 and 0.5 g-at NH₄–N l^-1. Kinetic data derived from the V _e segment of the uptake curve for silicate-limited populations yielded different values of V _max and K _s for each of the three species. In a number of parameters that were measured, T. gravida was clearly different from C. debilis and S. costatum and its recovery from nutrient starvation was the slowest. Recovery of all species from silicate limitation or starvation was slower than from ammonium limitation or starvation. Ammonium-starved populations maintained a maximal uptake rate at a substrate concentration an order of magnitude lower (0.1 g-at NH₄–N l^-1) than that observed for NH₄-limited populations (1.0 g-at NH₄–N l^-1). Adaptation to the severity of the nutrient limitation occurred as changes in the magnitude of cellular characteristics, such as short-term uptake potential (V _s ) and affinity for the substrate (K _s ). The consequence of these results are discussed in terms of another possible mechanism to explain changes in species composition and succession in nutrient-depleted environments.Contribution No. 944 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Microsatellite variation, effective population size, and population genetic structure of vermilion snapper, Rhomboplites aurorubens, off the southeastern USA

Vermilion snapper (Rhomboplites aurorubens) were collected from four sites off the Atlantic coast of the USA and one site in the Gulf of Mexico to evaluate effective population size and genetic stock structure. Previous studies had suggested geographic variation in the ratio of males to females, so this population characteristic was explored in conjunction with the genetic analysis. Sex ratio varied greatly among the five sample sites; males comprised 57% of samples in the Gulf of Mexico, while within the South Atlantic Bight they comprised between 36% (Morehead City, North Carolina) and 53% (Carolina Beach, North Carolina) of samples. No clear geographic trends in the sex ratio emerged; instead, it was found to vary with fish length, the percentage of males decreasing with increasing size. Allelic variation assessed at seven dinucleotide microsatellite loci was large; gene diversities ranged from 0.43 to 0.95 and allelic counts from 7 to 39. Estimates of the effective population size ranged from 24 500 (based on the infinite-alleles model) to 150 500 (based on the stepwise-mutation model). There was evidence for excess homozygosity within samples: estimates of F _IS (the correlation of alleles within individuals) ranged from 0.01 to 0.03 among the seven loci, and three estimates were significantly greater than zero. Differentiation among localities was very weak, as estimates of F _ST (the correlation of alleles within populations) were on the order of 0.001 to 0.002 and genetic distance estimates between localities were not related to geographic distances. This suggested that vermilion snapper in the South Atlantic Bight (Cape Hatteras, North Carolina to Cape Canaveral, Florida) and Gulf of Mexico are likely to consist of one genetic stock. Despite the overall homogeneity, there were indications of a temporally dynamic local structure that would bear further examination. Received: 6 July 1998 / Accepted: 9 February 1999